#030 Appropriate Pre-drying of Plastic Molding Materials (Revised Version)
Usually, the plastic materials are formed in the shape of pellets and sent from the raw material manufacturers in paper bags, etc.
Since these pellets would have absorbed the moisture in the atmosphere, if they are used for injection molding while they still contain a lot of moisture, depending on the type of material, they can undergo hydrolysis, or their physical properties can decrease. Further, it is possible for silver streaks to appear on the surface of the molded product, and it is also possible for short shots or burn due to gas to occur.
In view of this, in the case of most molding materials, before they are put into the hopper drier, it is necessary to carry out pre-drying in a box type drying oven.
In pre-drying it is recommended to observe the appropriate drying temperature and drying time. This is because, if the drying is done at a temperature less than the appropriate temperature, even if the drying is done for a long time, the moisture content cannot be removed as desired. A material whose pre-drying has been completed should be used as quickly as possible. When any left over material is to be used some days later, carry out its pre-drying again before use.
The pre-drying conditions of special plastics are listed in Table 1.
Table 1 Pre-drying temperatures of plastic molding materials
Pellets of plastic molding materials generally absorb moisture from the atmosphere to a certain extent. If the quantity of absorbed moisture is large, the plastic can undergo hydrolysis (there are plastics that undergo chemical dissociation with water as the initiator) in the process of being melted and mixed in the cylinder of the injection molding machine, or, when molding is done, this can cause silver streaks, air bubbles, or glossiness defects on the surface of the molded product, or can cause copying defects, etc. Therefore, it is necessary beforehand to put the pellets of molding materials in a drying apparatus and remove the moisture content in them. If the pre-drying is not done appropriately, it can lead to variations in the fluidity, deterioration of physical characteristics, and molding defects.
The following are the main types of driers being used at present.
(1) Hot air drier
The hopper drier and the box type drier are the typical equipment used with this type. The drying method is that of blowing hot air at the pellets thereby evaporating the moisture in them. Although this is a common and simple drying method, this method is not suitable for completely removing the moisture content.
(2) Dehumidified hot air t drier
In this method, after first removing the moisture in air, that air is heated and blown on the pellets thereby evaporating the moisture content in them. Since the air used for drying is re-circulated and used again after being dehumidified, heat loss will be small, and it is possible to carry out rational drying. This method is suitable for drying PBT, etc.
(3) Reduced pressure heat transfer type drier
This is a method for evaporating the moisture content in the pellet by heat transfer in a reduced pressure environment. Drying at low temperatures becomes possible, and hence it is possible to prevent the oxidization of plastic and to reduce the effects of additives in the pellet. In addition, this is also a method in which thermal loss is also small. This type of drier is attracting a lot of attention as the drier of the future.
|*||Reference:"Injection Molding Dictionary", p. 214, Peripheral and accessory equipment (Hideki Kubo, Industry Research Institute, (2002))|